There are lots of systems to develop between the low level generic engine and the high level one-off (ish) gameplay code for a specific product. For example, a

There are lots of systems to develop between the low level generic engine and the high level one-off (ish) gameplay code for a specific product. For example, a good brawler combat engine and the tools, pipeline and experience to put it to good use is pure gold to a company like Platinum Games. You can see similar systems lineages in companies attached to certain game types, like Bungie, Firaxis, etc.